You can use a critical hit from one weapon to create an opening for another.
Benefit: When you are able to make an attack with each hand (as a result of either fighting with two weapons, having a hand free to make unarmed attacks, or having the Improved Shield Bash feat) and your primary weapon scores a critical hit, you can make an attack with your off-hand against the same target as an immediate action, without taking any two-weapon attack penalties. You gain a +1 bonus on your attack roll for this attack if the weapon with which you scored the critical hit has greater than a x2 critical multiplier.
The Genius Guide To: Feats of Critical Combat. Copyright 2010, Super Genius Games. Author: Owen K.C. Stephens